Self-shielding type cyclotron

ABSTRACT

Disclosed is a self-shielding type cyclotron which makes it unnecessary to enclose the cyclotron by an extra shielding structure to protect personnel or equipment from radiation injury or damage.

United States Patent [191 Karasawa Nov. 18, 1975 154] SELF-SHIELDING TYPE CYCLOTRON [75] Inventor: Takashi Karasawa, Tokyo, Japan [56] Refmnces Cited [73] Assignee: Rikagaku Kenkyusho, Wako, Japan UNITED STATES PATENTS 2,872,574 2/1959 McMillan et a1. 338/234 [22] F11C1I NOV. 21, 1973 3,175,131 3/1965 :Burleigh et a1. i. 313/62 X 21 Appi. No.: 417,929

Primary Examiner-Pam L. Gensler [30] Foreign Application Priority Data ABSTRACT Dec. 4, Japan s ose is a self Shieldin g type cyclotron which Nov. 2, 1973 Japan 48-123629 makes it unnecessary to enclose the Cyclotron y an extra shielding structure to protect personnel or 3l3/g2dsi2ig2/gg equipment from radiation injury or damage [58] Field of Search 313/62; 328/234 2 Cla m 6 Drawing Figures 2 i 19 l 27 9 Y u .1 2| 2 Y US. Patent Nov. 18,1975 Sheet1of3 3,921,019

FIG.| PRIOR ART FIG.2 PRIOR ART US. Patent Nov. 181 1975 Sheet20f3 3,921,019

FIG.4

1 SELF-SHIELDING TYPE cYc LoTRoN" This invention relates to an improvement in; or relating to a cyclotron, and more-particularly to a-.-selfshielding type cyclotron which makes it unnecessaryto enclose the cyclotron by an extra shielding structure to protectpersonnel or equipment from radiation injury or damage. I t I Referring to FIGS. 1 and 2, there is shown a-conventional cyclotron in which a vacuum casing ,3 contains two accelerating electrodestcalled dees) 2, and is disposed between thepoles l of a magnetstructure. Also, as shown in the drawings, in the opening defined by the oppositeyokes9 of the magnet structure there are together disposed a high-frequency,powerinlet4 to the dees, an exhaust 6 to a vacuum purnp and anoutlet 8 for extracting ions. The ions are accelerated and deflected by a deflector 7.. Thus, theopening defined. by.t he magnet'yokes 9,,will leakradiation whilelthecyclotron is running, and therefore it is necessary that an extra shield structure to .surround the cyclotron be providedto protect personnel or equipment from radiation injury or damage. Inthis connectiori the' floor space required for installation of the cyclotron is as large as the floor space of the cyclotron per se plus that ofgthe shielding structure. Therefore, ininstallin a cyclotron, extra space and,cost is ineyitable. Recently-a different type cyclotron has been proposed, in which'a high frequency electric power issuppliedto "the'clees viaa'c o-I axial unit mounted at'the center of the magnet pole. A'

cyclotron of this type has a resonato'r an exhaust, an ion outlet and other accessaries disposed. in the" epening of the magnet structure, and therefore it also necessitates a radiation-shielding structure.

, The object of this invention is to provide a selfshield ing type cyclotron which is compact and less expensive,

and facilitates installation thereof; Other objects and advantages of thisinvention 'will be apparent from the following descriptien'which is "made with reference to the accompanying drawingsihwhicjh:

FIG. 1 is a plan view of a tiallybroken; I ,5

FIG. ,2 is a side view of the conventional cyaieaenas shown in FIG. l; e Y f FIG. 3 is a vertical section of a cyclotron accordin gto this invention; f a v I i I FIG. 4 is a cross section of the cyclotron, taken along the line o-.o in FIG.' 3;and

conventional cyclotron, par- FIGS. Sand 6 are similar to FIGS. "3 and'4, but showing another embodiment of this invention;

Referring to FIGS. 3 and 4, a cyclotron according to this invention comprises a pair of annular bodies A, A, integrally assembled in the over laying relation. An annular body has a center leg portion 11 (hereinafter referred to as pole) and a peripheral leg (hereinafter referred to as yoke), thus taking a form of the letter E in vertical section. In the drawings, 13 is an electromagnet coil (called Bitter type electromagnet); 15 dees positioned in the first space 14 defined by the opposite poles 11; 17 a target box mounted in the second space 16 defined by the opposite magnet coils 13; 19 coaxial elementsof the resonator disposed in the central apertures 18 of the poles and connected to the dees; and 20 a wall for keeping the first space 14 vacuum-tight. 21 is a short-circuit plate of the coaxial elements; 22 a beam deflector; 23 is an ion source 24 a vacuum-tight gasket; 25 linear shafts which allow a hyvbody A; and finally 26 an exhaust duct to the. vacuum pump- (not shown). I l

As shown in these drawings a cyclotron is consisted of a pair of annular counterparts integrally connected in a vacuum-tight-fashion. Within this vacuum-tight assembly there are disposed a vacuum box, a target box and other accessories. Thanks to this unique structure, and if the wall of the yoke 10 has a thickness enough to prevent radiation from passing through the yoke. little or no radiation will appear outside of the cyclotron.

Two coaxial elements 19 of the resonator are inserted in the center apertures 18 of the opposite poles 11. This'center aperture extends over. a relatively long distance, compared with the diameter of the aperture. Therefore, it will leak no appreciable amount of radiation. As to the exhaust .26 to the vacuum pump, it extends through the yoke 10 in the form of labyrinth, and therefore it will not leak an appreciable amount of radiation, either. The lead wires to the coils of the electromagnets 13 and the gas supply pipes to the ion source are put in the apertures :of such a small diameter that they may leak a negligible small amount of radiation.

In this particular embodiment a split type dee is used,

and; therefore when-putting the upper annular body A on the. lower one A, onecounterpart of the dee mounted in the upper annular body A must accurately. mate with the other counterpart of the dee housed in the lower annular body A. Any misalignment of these counterparts will cause unstable operation in the resonator.

:NOW, the shielding capability .of a cyclotron according to this invention will be discussed, taking the operating condition of 4 MeV, 10 uA deutron as an example. 'y-ray and neutron will be generated when deutron is accelerated in the cyclotron. However, if the shielding against neutron is satisfied, then the shielding against y-ray will suffice. In this connection the shielding capability; against neutron is handled in the following. In the event of bombardment against a Be target by the deutron as specified above, a large number of neutrons will be produced, and. for the operating condition above mentioned the number of neutrons thus gener .ated is:. 1

10 x 10-" a 116 x 10- X 10 5 X low/Sec At'a-one meter distance from the Be target the amount of radiation is 4 X l0 /cm sec. As is well known, the dose of neutrons (in terms of rem") depends on the energy of neutron, l-mr is equal to about IO/cm for thermal neutrons, and is equal to about I0"/cm for 100 'KeV fast neutrons. For the approximate estimation thermal neutron is percent of the total number of neutrons, and KeV fast neutron is 20 percent. Therefore the dose rate of thermal and fast neutrons is 1.1 mr/sec. In case of targets other than the Be target, such as CO N or H O target, the rate of generation of neutrons is approximately one tenth of that in case of the Be target, i.e., about 0.1 mr/sec.

Suppose that the cyclotron is running for 30 hours per week. In case of the Be target the neutrons will be about l X 10 mr/week, whereas in case of targets other than the Be target the neutrons will be about I X 10 mr/week. The allowable dose at the boundary of the controlled area is 30 mr/week.

Suppose that the yoke of the assembly is made of an iron as thick as 30 cm. Most neutrons-take an oblique path in passing through the yoke wall, and therefore effective thickness of the yoke wall is as large as about 45 cm. As is well known, an iron sheet as thick as 15 cm will reduce the neutron flux 10 times. Therefore. a 45 cm-thick iron sheet will reduce the neutron flux l.000 times.

As for the above examples, in case of the Be target the radiation dose at a l m-distance will be 100 mr/week, whereas in case of targets other than the Be target the radiation dose will be 10 mr/week.

In case of the Be target the radiation dose is three times as large as the allowable value, and therefore the controlled area must extend and cover 2 meters around the cyclotron.

Alternatively, the cyclotron may be applied by a paraffin coating several centimeters thick. In case of targets other than the Be target, however, the radiation dosev is one third of the allowable value, causing no radiation injury ordamage.

Referring to FIGS. and 6, there are shown another embodiment of this invention. This cyclotron is structurally the same as the first embodiment of FIGS. 3 and 4 except for; the coaxial elements 19 of the resonator system extend through the yoke of the annular assembly to the dees l5, and the second space 16 is partially loaded with a shielding material 27 (such as paraffin or boron-and-paraffin mixture coated with a cadmium plate) around each coaxial unit.

This structure is advantageous to the integral connection between the dees l5 and the coaxial element 19, assuring a sufficient mechanical strength in the joint. Also, this arrangement permits the use of an integral type resonator in place of the split type-one, thus avoiding the unstable operation which would be caused by any misalignment of respective counterparts of the split type resonator when assembled.

The leak of the radiation from a target box can be prevented by putting a 30 cm-thick, cadmium-coated paraffin or paraffin-and-boron mixture layer on the coaxial element of the resonator in the second space. These particular shielding materials as thick as cm will decrease neutron flux 10 times. Suppose that on an average neutrons pass through the shield obliquely at and therefore the apparent and effective thickness is as large as 36 cm. Thus, the neutron flux will decrease 60 times. Suppose that the coaxial element of the resonator is, for instance, 15 cm in diameter and cm in length. The neutrons when passing through this cylindrical duct are supposed to decrease about 30 flO 4 times. As a total result the neutron flux will decrease l,800 (1/60 X 1/30 1/1800). This figure proves the satisfactory self-shielding attained in a cyclotron according to this invention.

As mentioned earlier. a cyclotron according to this invention is a closed structure, contrary to the conventional open-type cyclotron. Usually a 30 cm-thick yoke is adequate to prevent the leak of radiation. The thickness of the yoke increases with the energy of the cyclotron in designing.

The unique structure of this invention is advantageous particularly to a small-sized cyclotron having a target material other than Be (such as B, C, N, 0, Ne and other elements) for use in producing short-life radioisotopes for medical use, or in generating neutrons or charged particles for radiochemical analysis.

What is claimed is:

l. A cyclotron comprising: a pair of annular counterpart bodies overlying and integrally assembled with each other, each said annular counterpart body having, in section, a center leg comprising a pole and two side legs comprising a yoke in the form of the letter E, said yoke having a thickness adequate to prevent radiation from passing therethrough; electromagnet coils positioned around said pole and surrounded by said yoke; dees positioned in the first closed space defined by the opposite end surfaces of said pole; a target box positioned in the second space defined by the opposite surfaces of said electromagnet coils; a resonator comprising co-axial means inserted axially in central apertures of said poles and integrally connected to said dees; and gas-tight means for keeping vacuum at least in said first closedspace.

2. A cyclotron comprising: a pairof annular counterpart bodies overlying and integrally assembled with each other, each said annular counterpart body having, in section, a center leg comprising a pole and two side legs comprising a yoke in the form of the letter E", said yoke having a thickness adequate to prevent radiatiop from passing therethrough; electromagnet coils positioned around said pole and surrounded by said yoke; dees positioned in the first closed space defined by the opposite end surfaces of said pole; a target box positioned in the second space defined by the opposite surfaces of said electromagnet coils, a resonator comprising co-axial means extending transversely through said yokes and intergrally connected to said dees; radiation shield means positioned around said co-axial means in said second space; and gas-tight means for keeping vacuum at least in said first closed space. 

1. A cyclotron comprising: a pair of annular counterpart bodies overlying and integrally assembled with each other, each said annular counterpart body having, in section, a center leg comprising a pole and two side legs comprising a yoke in the form of the letter ''''E'''', said yoke having a thickness adequate to prevent radiation from passing therethrough; electromagnet coils positioned around said pole and surrounded by said yoke; dees positioned in the first closed space defined by the opposite end surfaces of said pole; a target box positioned in the second space defined by the opposite surfaces of said electromagnet coils; a resonator comprising co-axial means inserted axiaLly in central apertures of said poles and integrally connected to said dees; and gas-tight means for keeping vacuum at least in said first closed space.
 2. A cyclotron comprising: a pair of annular counterpart bodies overlying and integrally assembled with each other, each said annular counterpart body having, in section, a center leg comprising a pole and two side legs comprising a yoke in the form of the letter ''''E'''', said yoke having a thickness adequate to prevent radiation from passing therethrough; electromagnet coils positioned around said pole and surrounded by said yoke; dees positioned in the first closed space defined by the opposite end surfaces of said pole; a target box positioned in the second space defined by the opposite surfaces of said electromagnet coils, a resonator comprising co-axial means extending transversely through said yokes and intergrally connected to said dees; radiation shield means positioned around said co-axial means in said second space; and gas-tight means for keeping vacuum at least in said first closed space. 